Water

Question 1

Name two chemical tests for the presence of water and their colour changes

Answer 1

Anydrous copper sulfate white –> blue Anydrous cobalt chloride blue –> pink

Question 2

What is hard water?

Answer 2

Water that does not lather easily with soap.

Question 3

Which ions are dissolved in hard water?

Answer 3

Calcium or Magnesium

Question 4

What forms when hard water reacts with soap?

Answer 4

An insoluble scum called calcium stearate Sodium stearate + calcium ions —> calcium stearate + sodium ions

Question 5

What is temporary hard water?

Answer 5

Hard water which can be softened by boiling.

Question 6

Give four physical properties of water?

Answer 6

Liquid at room temp and pressure Colourless Melting point 0°C Boiling point 100°C

Question 7

How does temporary hard water form?

Answer 7

Carbon dioxide dissolves in rain water forming a weak carbonic acid. CO2 + H2O –> H2CO3 Carbonic acid falls on limestone rock and forms soluble calcium hydrogen carbonate. H2CO3 + CaCO3 –> Ca(HCO3)2 Calcium ions are in the water

Question 8

What happens to temporary hard water when it is heated up?

Answer 8

It thermally decomposes forming insoluble calcium carbonate Ca(HCO3)2 –> CaCO3 + H2O + CO2

Question 9

What is permanent hard water?

Answer 9

Hard water which can not be softened by boiling.

Question 10

Which calcium or magnesium compounds form permanent hard water?

Answer 10

Calcium chloride or magnesium sulfate

Question 11

Give four advantages of hard water

Answer 11

Tastes good Contains calcium ions needed for strong bones and teeth Better for tanning leather Better for brewing beer

Question 12

Give four disadvantages of hard water

Answer 12

Wastes soap Formation of scale in pipes and kettles Expensive - dishwasher salt required to prevent lime scale in dishwashers

Question 13

Give three methods of softening hard water

Answer 13

Boiling (temporary hard water only) Washing soda Ion exchange column

Question 14

Explain how washing soda removes hardness in water.

Answer 14

The calcium ions from the hard water combine with carbonate ions from the washing soda (sodium carbonate) to form insoluble calcium carbonate. Ca2+ + CO32- –> CaCO3(s)

Question 15

Explain how an ion exchange column removes hardness in water.

Answer 15

The ion exchange column contains a resin with sodium ions attached. When the hard water is poured over the calcium ions are replaced by sodium ions.

Question 16

What effect does hard water have on detergents? e.g. Cillit Bang

Answer 16

No effect

Question 17

What is a precipitation reaction?

Answer 17

A reaction where ions from two solutions react together to make an insoluble substance.

Question 18

What does a deliquescent substance do?

Answer 18

Absorbs moisture and dissolves in the moisture making a solution.

Question 19

What is a dessicant?

Answer 19

A substance which absorbs moisture from the air.

Question 20

Give an example of a dessicant substance and where it is used.

Answer 20

Silica gel used in packaging of electrical equipment e.g. Camera

Question 21

Give an example of a deliquescent substance

Answer 21

Calcium chloride used in the rusting experiment to remove moisture.

Question 22

Why is fluoride added to water supplies.

Answer 22

Increases the resistance of teeth to attack from acid.

Question 23

Give some advantages of adding fluoride.

Answer 23

Reverses the effects of loss tooth enamel Increases resistance of teeth to acid decay Ensures all the population has access to the benefits of fluoride

Question 24

Give some disadvantages of adding fluoride to water

Answer 24

High concentration leads to dental fluorosis Difficult to monitor dosage as people drink differing amounts of water or eat foods high in fluoride Costly

Question 25

What are the public’s concerns with use of barium salts in medicine?

Answer 25

Barium is toxic and could end up in the water supplies.

Question 26

Why is barium Sulfate still used in X-rays?

Answer 26

It is insoluble (white precipitate) so unlikely to end up in water supplies.